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THE CORRELATION BETWEEN THE LOWER INCISOR ANGLE AND STABILITY Binh N. Tran, D.D.S. An Abstract Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2007 ABSTRACT Lateral cephalograms and study casts of 62 patients with Class I and II malocclusions were evaluated to determine if any relationships exist between the change in lower incisal to mandibular plane angle and occlusal stability one year after treatment. A statistically significant but weak correlation (r=0.328, p<0.01) was found between the changes in the lower incisal angle and irregularity index one year posttreatment. However no significant difference in incisor irregularity was found when the sample was broken down into subgroups of cases finished with <4° change and cases with ≥4° change in lower incisor angle. In addition, no difference was observed whether the cases where finished with lower incisor angle with Tweed’s recommended range (85°-93°) or not. 1 THE CORRELATION BETWEEN THE LOWER INCISOR ANGLE AND STABILITY Binh N. Tran, D.D.S. A Thesis Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2007 COMMITTEE IN CHARGE OF CANDIDACY: Associate Professor Ki Beom Kim Chairperson and Advisor Assistant Professor Maria Atique Assistant Professor Gus Sotiropoulus DEDICATIONS To my wonderful family: My beautiful wife, Michele My curious son, Maximus My precious daughter, Audrey Thank you for your being part of my life and giving me the motivation to finish this project. ii ACKNOWLEDGEMENTS I would like to acknowledge the following individuals: Dr. Ki Beom Kim for chairing my thesis committee. You have been a great mentor. Thank you for your guidance and your time in the development and writing of this thesis. Dr. Gus Sotiropoulos for serving on my thesis committee. You have been a great teacher. Thank you for your time in the development and writing of this thesis. It has been a pleasure to know you and to work with you. Dr. Maria Atique for serving on my thesis committee. It has been a privilege to work with you and I appreciate your guidance throughout the research process. TABLE OF CONTENTS iii List of Tables............................................v List of Figures..........................................vi CHAPTER 1: INTRODUCTION..................................1 CHAPTER 2: REVIEW OF THE LITERATURE Irregularity Index (II).......................4 Normal Development............................5 Pretreatment Factors..........................6 Extraction and Nonextraction..................6 Late Mandibular Growth.......................10 Third Molars.................................10 Transeptal Fibers............................10 Early Versus Late Treatment..................12 Tooth Dimension..............................13 References...................................17 CHAPTER 3: JOURNAL ARTICLE Abstract.....................................24 Introduction.................................25 Materials and Methods........................32 Selection Criteria........................32 Cepahalometric Analysis...................33 Model Analysis............................33 Statistical Analysis......................34 Results......................................35 Discussion...................................38 Comparisons with Previous Studies.........38 Limitations...............................42 Conclusions..................................42 Literature Cited.............................44 Vita Auctoris............................................48 iv LIST OF TABLES Table 1: A Review of the Literature...................31 Table 2: Analysis of Error............................35 Table 3: Mean ± SD of the Sample and Subgroups........36 Table 4: Comparison between Groups within Tweed Norms (85°-93°) and Other IMPAs.....................37 Table 5: Comparison between Groups with Incisal Changes < and ≥ 4°...................................37 v LIST OF FIGURES Figure 1: Irregularity Index...........................34 Figure 2: Correlation between the Lower Incisal Angle and Irregularity Index...........................40 vi CHAPTER I: INTRODUCTION The four objectives that many orthodontists strive to attain are health, function, aesthetics, and stability of the occlusion. Of these four objectives, stability is the most difficult to maintain. There are many protocols or techniques,1 both old and new, that today’s orthodontist can choose to achieve a stable result. However, the efficacy of the chosen plan of treatment remains questionable because of the lack of agreement in the literature regarding a stable occlusion.2 Before orthodontics was a specialty, Norman Kingsley proposed that a good occlusion is the most important factor in determining a stable occlusion.1 Charles H. Tweed, prescribed to the idea that the mandibular dental arch, acting as a template for the maxillary arch, plays a key role in stability. Based on seventeen years of “factual” clinical evidence gathered from his practice, Tweed interpreted Edward H. Angle’s definition of “the line of occlusion” as laying the teeth in a functional, mechanical balanced axial inclination over alveolar bone, especially the mandibular incisor.3 Furthermore, Tweed supports this claim by noting that approximately 20% of all his cases that he considered 1 successful had an incisal to mandibular plane angle ranging about 8° from 85° to 93°. This range of lower incisal angle is also corroborated by Allan Brodie4 and Holly Broadbent5 who published papers on dentofacial pattern of untreated children with an average lower incisor angle of 87.9º, respectively. 88.3º and Historically, the lower incisal angle has been considered a major factor in the treatment objectives and is synonymous with the Tweed philosophy of treatment. Another major concept on stability is the Equilibrium Theory that proposes the dentition is kept in a balanced position by its environment (i.e. perioral muscles, tongue, transeptal ligaments, etc.). In theory, altering the oral environment will cause a change in the occlusion; therefore to create a stable occlusion, teeth should be place in a position balanced by the forces around it. Many authors tried to explain this theory by citing clinical cases of muscular abnormalities causing malocclusion due to an imbalance of forces on the dentition.6-8 However, there is a lack of direct evidence to support this theory.9 In orthodontics, it is desirable to keep the treatment results as stable as possible for the lifetime of the patient. Crowding of the lower incisors can be 2 corrected by a few general methods: extract teeth to gain space, procline the incisors, perform interproximal reduction, or expansion. Each of these treatment options can affect the lower incisor angle cephalometrically and potentially alter any equilibrium that may have existed before. The purpose of this retrospective study is to determine whether changing the lower incisor angle during treatment affects the stability of the occlusion after orthodontic treatment. 3 CHAPTER 2: REVIEW OF THE LITERATURE Irregularity Index There are many factors that may play a role in stability. To test these factors, there must be a method to record stability. Because a change from good occlusion to malocclusion is usually expressed early as mandibular incisor crowding, a change in lower incisor crowding over time is often used to measure stability.10-13 In 1975, Robert Little proposed and tested a reliable and valid method of scoring the lower incisor crowding called the irregularity index, which relies on a digital caliper to measure the total displacement of the anatomic contact points of the lower anterior teeth.14 This method is often used to compare the changes in crowding from posttreatment to postretention.10-12,15 By comparing the change in irregularity index between groups with different treatments rendered, an investigator can compare the relative difference in stability. 4 Normal Development of the Occlusion Before we can begin to understand the stability of a corrected occlusion, it is prudent to examine the trends and normal development of untreated occlusions. Sinclair and Little13 evaluated dental casts of 65 untreated normal occlusions at three different dental stages (mixeddentition, early permanent dentition, and adult dentition) and reported the following findings: (1) arch length (Figure 1) decrease from mixed dentition into early adulthood, (2) intercanine and intermolar width is relatively stable with statistically significant decrease occurring in females from 13 to 20 years, (3) overjet and overbite increased from 9 to 13 years and then decrease from 13 to 20 years, resulting in minimal net change, and (4) incisor irregularity increase from 13 to 20 years, slightly more irregularity for females. consistent with previous research.16,17 These findings are Richardson et al.18 and Bishara et al.19 evaluating crowding of the lower incisors in untreated occlusion after age 18 found that crowding continues into the third and fourth decade of life, respectively. A difference is that the late adult crowding is minimal, often less than 1 mm over 10 years.18,19 5 Pretreatment Factors Many studies20-22 have attempted to evaluate possible factors associated with a stable occlusion. In 1981, Little et al.15 did a retrospective study using 65 first premolar extraction cases which had undergone routine edgewise orthodontic therapy followed by retention and at least 10 years removal of all retention devices. No significance differences in irregularity index at the pretreatment or postretention period were found between Angle malocclusion classes, sexes, ages, and arch width change at the canines or molars.15 All factors had a correlation coefficient less than r=0.38.15 Arch width measured across the mandibular canine teeth typically reduces posttreatment whether the case was expanded during treatment or not.11 Little also observed that cases with minimal pretreatment crowding was worse at postretention, while severe initial crowding showed improvements in postretention. Extraction versus Nonextraction As a continued study of Little’s report, Shield and colleagues23 assessed the records of 54 first-premolar 6 extraction cases treated with traditional edgewise therapy and were at least 10 years out into postretention, where patients stopped using any forms of retainer. The purpose of this study was to appraise the cephalometric records taken at pretreatment (T1), posttreatment (T2), and postretention (T3) for any variables that may be correlated with stability and mandibular anterior alignment. All cases had a posttreatment irregularity index of less than 3.0 mm. Treatment (T1-T2) and postretention (T2-T3) changes in incisor axial inclinations had slight correlation coefficients for ı to NA° (r<-0.55, p≤0.01) and ī to NPg° (r=-0.58, p≤0.01). The data suggested a slight tendency for incisor inclination to return toward the pretreatment value during the postretention period. Furthermore, they found no significant association between any specific pre- and postreatment cephalometrics parameters such as incisor position and skeletal development with long-term mandibular irregularity. However, Shield suggests, “It is conceivable that there exists an envelope in which tooth movement may be accomplished without significant inclination relapse.” 23 Due to the selection bias of his sample, Shield is not able to compare the various incisor angulations changes and its association with postretention irregularity. 7 Similar to Shield’s findings, a long-term postretention (at least 10 years) study24 from Baylor University on Class I premolar extraction treatment using the Tweed edgewise treatment philosophy found low correlation (r≤±0.57) between postretention irregularity index and any of the following parameters: intermolar widths, pretreatment mandibular incisor inclination, incisor proclination during treatment, and posttreatment irregularity. width. No correlation was reported for intercanine The cases in the Baylor study had postretention relapse comparable to that reported in the study of untreated normal occlusions by previous authors.13 However in this study, using their described Tweed treatment philosophy, the results of the study indicated satisfactory long-term stability (irregularity index <3.5 mm) in 80% of the treated patients as compared to Little’s15 finding of less than 30% of his cases, which included Class II and I malocclusions. The authors24 suggested that minimal lower incisal angle change influenced their success. An alternative reason for their findings discrepancy is that the Baylor study had an average of 1.8 mm of posttreatment irregularity and Little may have had a greater average, which he did not report, but can be inferred from a 8 continuation of his study by Shield and colleagues23 who reported cases of postreatment irregularity up to 2.9 mm. In a study of 30 Class I malocclusions treated nonextraction, Weinberg and Sadowsky,25 based on strength of the correlation (p<0.05), determined that pretreatment lower incisor crowding is moderately associated with increased posttreatment arch length (r=-0.68) and slightly associated with arch depth (r=-0.55), interfirst-premolar width (r=-0.45), intermolar width (r=-0.45), and intersecond-premolar width (r=-0.36). The correlations from this study suggest that pretreatment crowding is resolved primarily by increasing arch length via proclining the lower incisor or expanding the arch. Comparing nonextraction and extraction treatment effects on long-term stability is intuitively better tested by comparing similar pretreatment malocclusion. Paquette, Beattie, and Johnston26 did a long-term comparison study of nonextraction and premolar extraction edgewise therapy in “borderline” Class II patients and reported no difference in posttreatment irregularity. This conclusion is also corroborated by other studies10,27 comparing lower incisor irregularity between extraction and nonextraction cases. 9 Post-Puberty Mandibular Growth Paquette and associates26 also found a moderate correlation (r=-0.75, p<0.01) between the difference in growth between the jaws and lower incisor movement posttreatment. The authors26 argue that relapse/settling of the occlusion is due to growth after treatment and that any changes made during treatments have little effect on destabilization of the occlusion as seen on an untreated normal occlusion.13 It is evident that the dentition changes with growth18,19 and that facial growth continues throughout a person’s life.19,28 Therefore, it is possible that the moderate correlation of differential jaw growth and dental changes in posttreatment are coincidental events with no relationship. Third Molars The effects of third molars on late lower arch crowding continue to be controversial. Richardson29 concluded that during the teenage years, pressure from the back of the third molar plays some role in the cause of late mandibular incisor crowding. Several studies30-32 were unable to demonstrate that third molars exert pressure on 10 the teeth mesial to them. Zachrisson33 in a recent review of the literature concluded that the current evidence and any influence that the third molar may have on lower incisor crowding does not justify the trauma that the patient must endure from extracting third molars. Transseptal Fibers A histological study on transseptal fibers and relapse following bodily retraction of teeth on eight Macaca rhesus monkeys showed that teeth with transseptal fibers removed before moving into an extraction space had virtually 0 mm of relapse (returning to their original position) while the control with intact transseptal fibers showed about 50 percent relapse in the first 12 hours.34 To apply this theory of periodontal fiber influence on orthodontic relapse in humans, a circumferential supracrestal fiberotomy (CSF) technique was developed by Edwards35 to surgically transect the transseptal and the epithelial tissue surrounding the tooth. A long-term prospective evaluation of CSF in alleviating orthodontic relapse was later done by Edwards36 showed that the CSF procedure significantly reduced relapse and was more effective in rotational than labiolingual relapse. 11 The study reports a mean mandibular irregularity of 3.12 mm for the control group (n=22) and 2.01 mm of irregularity for the CSF group (n=26) over a period of approximately 8 to 11 years postretention. Early Versus Late Treatment One of the reasons for early treatment as suggested by Lee and Dugoni37,38 is that early mixed dentition treatment using leeway space may improve lower incisor stability. In a study of nonextraction cases, Dugoni and associates39 propose that early treatment of anterior crowding with a passive lingual arch allow teeth to self correct and supracrestal fibers could reorganize around the incisors aligned position. In a study of early (mixed dentition) versus late treatment (permanent dentition)of crowded extraction cases, Haruki and Little40 found a significant mandibular irregularity index difference at postretention between the groups. In that study, all the patients had first premolar extraction with the difference being the time of premolar extraction either in the mixed dentition or permanent dentition. The early treatment group’s lower anterior teeth were aligned using fixed appliances. No serial extraction cases were included in 12 the study. In contrast, when Little and associates41,42 compare the incisor irregularities between the early treatment with serial extraction and late treatment with permanent teeth extraction, they found no difference in postretention irregularity index. In these studies41,42 the patients who had serial extraction, their teeth were allowed to undergo physiologic drift and then treated with fixed appliance later during comprehensive treatment. In summary, early treatment appears to be only significant if the lower anterior teeth are aligned before comprehensive treatment; perhaps this early alignment gives the supracrestal fibers time to reorganize around the straight teeth before comprehensive treatment. Tooth Dimension Studying tooth size and dimension began as early as the 1800s with GV Black43, who established a data base of tooth size means and Ballard44 in 1944 who found that 90% of his subjects had right-left tooth width asymmetry. Then in 1958, Bolton45 performed a tooth size study on fifty-five cases with excellent occlusions. The mesiodistal tooth width was measured on all the teeth on each cast from first molar to contralateral first molar. 13 The sum of the widths of the twelve mandibular teeth were divided by the sum of the widths of the twelve maxillary teeth and then multiplied by 100 to arrive at an overall ratio. The same method was used to set up an anterior ratio, consisting of the sum of the widths of the anterior six teeth. Bolton found the overall ratio average to be 91.3±0.26 and the anterior ratio average to be 77.2±0.22. This tooth size data was found to be closely related to that published by both Black and Ballard. Disharmonies in tooth size can be corrected by extraction of a tooth or teeth, placement of overcontoured restorations, or removal of tooth structure by interproximal reduction. There are some orthodontists who believe that tooth dimension is related to stability. Peck and Peck46 studied the lower incisal dimensions of 45 white female subjects with untreated perfect mandibular alignment and 70 subjects of comparable age and not selected as part of the perfect mandibular incisor group. They found that the mean mesiodistal (MD) crown diameters for the mandibular central and lateral incisors in the perfect-alignment group were smaller than the control population and the mean faciolingual (FL) crown diameters for the mandibular central and lateral incisors were larger in the perfect alignment sample than in the control population sample. 14 Based on these findings, Peck and Peck47 introduced an index (MD/FL x 100) to define well proportioned teeth. The MD/FL ratios for the central and lateral lower incisors are 88.4±4.3 and 90.4±4.8, respectively. To correct any ratio in excess of 100, the authors suggest interproximal reduction as a method to bring the lower incisors closer to ideal. The authors acknowledge that there are other factors that may influence lower incisor alignment; therefore it would not be strange to find crowded proportional sized incisors. To test Peck and Peck’s finding, Gilmore and Little48 did a retrospective postretention study, evaluating the 164 cases and found weak (r<0.60) to no correlation between the MD width or MD/FL ratio of the lower incisors and the lower incisor irregularities. These results were also corroborated by Shah and associates49 from the United Kingdom, who tried to improve measurements for the MD and FL widths by cross-sectioning the incisors on study cast and correlating the MD/FL ratio with the irregularity index. Other studies50,51 have failed to show any moderate correlation (r>0.70) between incisor morphology and incisor irregularity. One study by Rhee and Nahm52 found a moderate correlation coefficient of 0.77 between the ratio of the MD width at the incisal and cervical areas of the lower 15 incisors and incisor irregularity. The authors suggest that the area of contact between adjacent incisors is relevant to stability. However, it is important to note that the subjects for this study were Asians and that the other studies were Caucasians; the difference in race may play a role in the influence of the incisor anatomy and thus, result in different study outcomes. 16 References 1. Graber TM, Vanarsdall RL. Orthodontics: Current Principles and Techniques. St. Louis: C.V. Mosby; 2000. 2. Shah AA. Postretention changes in mandibular crowding: a review of the literature. Am J Orthod Dentofacial Orthop 2003;124:298-308. 3. Tweed CH. Indications for the extraction of teeth in orthodontic procedure. American Journal of Orthodontics and Oral Surgery 1944;30:405-428. 4. Brodie AG. Some Recent Observations on the Growth of the Face and Their Implications to the Orthodontist. American Journal of Orthodontics and Oral Surgery 1940;26:741-757. 5. Broadbent BH. Ontogenic Development of Occlusion. Angle Orthod 1941;11:223-241. 6. Eskew H, Shepard E. Congenital Aglossia. American Journal of Orthodontic 1949;35:116-119. 7. Brash JC, McKeag HT, Scott JH. The Aetiology of Irregularity and Malocclusion of the Teeth. The Dental Board of United Kingdom 1956:373-375. 8. Rogers AP. Making Facial Muscles our Allies in Treatment and Retention. Dental Cosmos 1922;64:711-730. 9. Proffit WR. Equilibrium theory revisited: factors influencing position of the teeth. Angle Orthod 1978;48:175-186. 10. Artun J, Garol JD, Little RM. Long-term stability of mandibular incisors following successful treatment of Class II, Division 1, malocclusions. Angle Orthod 1996;66:229238. 17 11. Little RM. Stability and relapse of dental arch alignment. Br J Orthod 1990;17:235-241. 12. Little RM, Riedel RA. Postretention evaluation of stability and relapse--mandibular arches with generalized spacing. Am J Orthod Dentofacial Orthop 1989;95:37-41. 13. Sinclair PM, Little RM. Maturation of untreated normal occlusions. Am J Orthod 1983;83:114-123. 14. Little RM. The irregularity index: a quantitative score of mandibular anterior alignment. Am J Orthod 1975;68:554563. 15. Little RM, Wallen TR, Riedel RA. Stability and relapse of mandibular anterior alignment-first premolar extraction cases treated by traditional edgewise orthodontics. Am J Orthod 1981;80:349-365. 16. DeKock WH. Dental arch depth and width studied longitudinally from 12 years of age to adulthood. Am J Orthod 1972;62:56-66. 17. Hummerfelt A, Slagsvold O. Changes in occlusion and craniofacial patterns between 11 and 25 years of age: a follow-up study of individuals with normal occlusion. Trans Eur Orthod Soc 1972:113-122. 18. Richardson ME, Gormley JS. Lower arch crowding in the third decade. Eur J Orthod 1998;20:597-607. 19. Bishara SE, Treder JE, Jakobsen JR. Facial and dental changes in adulthood. Am J Orthod Dentofacial Orthop 1994;106:175-186. 20. Little RM. Stability and relapse of mandibular anterior alignment: University of Washington studies. Semin Orthod 1999;5:191-204. 18 21. Littlewood SJ, Millett DT, Doubleday B, Bearn DR, Worthington HV. Orthodontic retention: A systematic review. J Orthod 2006;33:205-212. 22. Ormiston JP, Huang GJ, Little RM, Decker JD, Seuk GD. Retrospective analysis of long-term stable and unstable orthodontic treatment outcomes. Am J Orthod Dentofacial Orthop 2005;128:568-574; quiz 669. 23. Shields TE, Little RM, Chapko MK. Stability and relapse of mandibular anterior alignment: a cephalometric appraisal of first-premolar-extraction cases treated by traditional edgewise orthodontics. Am J Orthod 1985;87:27-38. 24. Boley JC, Mark JA, Sachdeva RC, Buschang PH. Long-term stability of Class I premolar extraction treatment. Am J Orthod Dentofacial Orthop 2003;124:277-287. 25. Weinberg M, Sadowsky C. Resolution of mandibular arch crowding in growing patients with Class I malocclusions treated nonextraction. Am J Orthod Dentofacial Orthop 1996;110:359-364. 26. Paquette DE, Beattie JR, Johnston LE, Jr. A long-term comparison of nonextraction and premolar extraction edgewise therapy in "borderline" Class II patients. Am J Orthod Dentofacial Orthop 1992;102:1-14. 27. Erdinc AE, Nanda RS, Isiksal E. Relapse of anterior crowding in patients treated with extraction and nonextraction of premolars. Am J Orthod Dentofacial Orthop 2006;129:775-784. 28. Behrents RG. Growth in the ageing craniofacial skeleton. Monograph 17, Craniofacial Growth Series. University of Michigan, Ann Arbor: Center for Human Growth and Development; 1985. 29. Richardson ME. The etiology of late lower arch crowding alternative to mesially directed forces: a review. Am J Orthod Dentofacial Orthop 1994;105:592-597. 19 30. Ades AG, Joondeph DR, Little RM, Chapko MK. A long-term study of the relationship of third molars to changes in the mandibular dental arch. Am J Orthod Dentofacial Orthop 1990;97:323-335. 31. Harradine NW, Pearson MH, Toth B. The effect of extraction of third molars on late lower incisor crowding: a randomized controlled trial. Br J Orthod 1998;25:117-122. 32. Kaplan RG. Mandibular third molars and postretention crowding. Am J Orthod 1974;66:411-430. 33. Zachrisson BU. Mandibular third molars and late lower arch crowding--the evidence base. World J Orthod 2005;6:180-186. 34. Parker GR. Transseptal fibers and relapse following bodily retration of teeth: a histologic study. Am J Orthod 1972;61:331-344. 35. Edwards JG. A surgical procedure to eliminate rotational relapse. Am J Orthod 1970;57:35-46. 36. Edwards JG. A long-term prospective evaluation of the circumferential supracrestal fiberotomy in alleviating orthodontic relapse. Am J Orthod Dentofacial Orthop 1988;93:380-387. 37. Dugoni SA, Lee JS. Mixed dentition case report. Am J Orthod Dentofacial Orthop 1995;107:239-244. 38. Lee JS, Dugoni SA. Mixed dentition treatment case report. Am J Orthod Dentofacial Orthop 1987;91:335-341. 39. Dugoni SA, Lee JS, Varela J, Dugoni AA. Early mixed dentition treatment: postretention evaluation of stability and relapse. Angle Orthod 1995;65:311-320. 40. Haruki T, Little RM. Early versus late treatment of crowded first premolar extraction cases: postretention 20 evaluation of stability and relapse. Angle Orthod 1998;68:61-68. 41. Little RM, Riedel RA, Engst ED. Serial extraction of first premolars--postretention evaluation of stability and relapse. Angle Orthod 1990;60:255-262. 42. McReynolds DC, Little RM. Mandibular second premolar extraction--postretention evaluation of stability and relapse. Angle Orthod 1991;61:133-144. 43. Black GV. Descriptive anatomy of the human teeth. Philadelphia,: The Wilmington dental manufacturing co.; 1890. 44. Ballard M. Asymmetry in Tooth Size: A Factor in the Etiology, Diagnosis and Treatment of Malooclusion. Angle Orthod 1944;14:67-71. 45. Bolton WA. Disharmony in tooth size and its relation to the analysis and treatment of malocclusion: University of Washington, 1952.; 1952: p. 40 p. 46. Peck S, Peck H. Crown dimensions and mandibular incisor alignment. Angle Orthod 1972;42:148-153. 47. Peck H, Peck S. An index for assessing tooth shape deviations as applied to the mandibular incisors. Am J Orthod 1972;61:384-401. 48. Gilmore CA, Little RM. Mandibular incisor dimensions and crowding. Am J Orthod 1984;86:493-502. 49. Shah AA, Elcock C, Brook AH. Incisor crown shape and crowding. Am J Orthod Dentofacial Orthop 2003;123:562-567. 50. Freitas MR, Castro RC, Janson G, Freitas KM, Henriques JF. Correlation between mandibular incisor crown morphologic index and postretention stability. Am J Orthod Dentofacial Orthop 2006;129:559-561. 21 51. Puneky PJ, Sadowsky C, BeGole EA. Tooth morphology and lower incisor alignment many years after orthodontic therapy. Am J Orthod 1984;86:299-305. 52. Rhee SH, Nahm DS. Triangular-shaped incisor crowns and crowding. Am J Orthod Dentofacial Orthop 2000;118:624-628. 22 THE CORRELATION BETWEEN THE LOWER INCISOR ANGLE AND STABILITY Binh N. Tran, D.D.S. Gus Sotiropolous, D.D.S., M.S. Maria Atique. D.D.S. Ki Beom Kim, D.D.S., M.S.D., Ph.D. An Abstract Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2007 CHAPTER 3: JOURNAL ARTICLE Abstract Lateral cephalograms and study casts of 62 patients with Class I and II malocclusions were evaluated to determine if any relationships exist between the change in lower incisal to mandibular plane angle and occlusal stability one year after treatment. A statistically significant but weak correlation (r=0.328, p<0.01) was found between the changes in the lower incisal angle and irregularity index one year posttreatment. However no significant difference in incisor irregularity was found when the sample was broken down into subgroups of cases finished with <4° change and cases with ≥4° change in lower incisor angle. In addition, no difference was observed whether the cases where finished with lower incisor angle with Tweed’s recommended range (85°-93°) or not. 23 Introduction The four objectives that many orthodontists strive to attain are health, function, aesthetics, and stability. Of these four objectives, stability is the most difficult to maintain. There are many protocols or techniques,1 both old and new, that today’s orthodontist can choose to achieve a stable result. However, the efficacy of the chosen plan of treatment remains questionable because of the lack of agreement in the literature regarding a stable occlusion.2 In orthodontics, it is desirable to keep the treatment results as stable as possible for the lifetime of the patient. Crowding of the lower incisors can be corrected by a few general methods that: extract teeth to gain space, procline the incisors, perform interproximal enamel reduction, or expansion. Each of these treatment options can affect the lower incisor angle cephalometrically and potentially alter any equilibrium that may have existed before. The purpose of this retrospective study was to determine whether changing the lower incisor angle during treatment affects the stability of the occlusion after treatment. 24 There are many factors that may play a role in stability. To test these factors, there must be a method to record stability. Because a change from good occlusion to malocclusion is usually expressed early as mandibular incisor crowding, a change in lower incisor crowding over time is often used to measure stability.3-6 In 1975, Robert Little proposed and tested a reliable and valid method of scoring the lower incisor crowding called the irregularity index, which relies on a digital caliper to measure the total displacement of the anatomic contact points of the lower anterior teeth.7 This method is often used to compare the changes in crowding from posttreatment to postretention.3-5,8 By comparing the change in irregularity index between groups with different treatments rendered, an investigator can compare the stability gain after treatment. Before we can begin to understand the stability of a corrected occlusion, it is prudent to examine the trends and normal development of untreated occlusions. Sinclair and Little6 evaluated dental casts of 65 untreated normal occlusions at three different dental stages (mixeddentition, early permanent dentition, and adult dentition) and reported the following findings: (1) arch length (Figure 1) decrease from mixed dentition into early 25 adulthood, (2) intercanine and intermolar width is relatively stable with statistically significant decrease occurring in females from 13 to 20 years, (3) overjet and overbite increased from 9 to 13 years and then decrease from 13 to 20 years, resulting in minimal net change, and (4) incisor irregularity increase from 13 to 20 years, slightly more irregularity for females. consistent with previous research.9,10 These findings are Richardson and Gormley11 and Bishara et al.12 evaluating crowding of the lower incisors in untreated occlusion after age 18 found that crowding continues into the third and fourth decade of life, respectively. A difference is that the late adult crowding is minimal, often less than 1 mm over 10 years.11,12 Many studies13-15 have attempted to evaluate possible factors associated with a stable occlusion. In 1981, Little et al.8 did a retrospective study using 65 first premolar extraction cases which had undergone routine edgewise orthodontic therapy followed by retention and at least 10 years removal of all retention devices. No significance differences in irregularity index at the pretreatment or postretention period were found between Angle malocclusion classes, sexes, ages, and arch width change at the canines or molars.8 All factors had a correlation coefficient less than r=0.38.8 26 Arch width measured across the intercanine teeth typically reduces posttreatment whether the case was expanded during treatment or not.13 Little also observed that cases with minimal pretreatment crowding was worse at postretention, while severe initial crowding showed improvements in postretention. As a continued study of Little’s report, Shield and colleagues16 assessed the records of 54 first-premolar extraction cases treated with traditional edgewise therapy and were at least 10 years out into postretention, where patients stopped using any form of retainer. The purpose of this study was to appraise the cephalometric records taken at pretreatment (T1), posttreatment (T2), and postretention (T3) for any variables that may be correlated with stability and mandibular anterior alignment. All cases had a posttreatment irregularity index of less than 3.0 mm. Treatment (T1-T2) and postretention (T2-T3) changes in incisor axial inclinations had slight correlation coefficients for ı to NA° (r<-0.55, p≤0.01) and ī to NPg° (r=-0.58, p≤0.01). The data suggested a slight tendency for incisor inclination to return toward the pretreatment value during the postretention period. Furthermore, they found no significant association between any specific pre- and postreatment cephalometrics 27 parameters such as incisor position and skeletal development with long-term mandibular irregularity. However, Shield and colleagues suggest, “It is conceivable that there exists an envelope in which tooth movement may be accomplished without significant inclination relapse.” 16 Due to the selection bias of their sample, Shield et al. was not able to compare the various incisor angulations changes and its association with postretention irregularity. In a study of 30 Class I malocclusions treated nonextraction, Weinberg and Sadowsky,17 based on strength of the correlation (p<0.05), determined that pretreatment lower incisor crowding is moderately associated with increased posttreatment arch length (r=-0.68) and slightly associated with arch depth (r=-0.55), interfirst-premolar width (r=-0.45), intermolar width (r=-0.45), and intersecond-premolar width (r =-0.36). The correlations from this study suggest that pretreatment crowding is resolved primarily by increasing arch length via proclining the lower incisor or expanding the arch. Comparing nonextraction and extraction treatment effects on long-term stability is intuitively better tested by comparing similar pretreatment malocclusion. Paquette, Beattie, and Johnston18 did a long-term comparison study of 28 nonextraction and premolar extraction edgewise therapy in “borderline” Class II cases and reported no difference in posttreatment irregularity. This conclusion is also corroborated by other studies3,19 comparing lower incisor irregularity between extraction and nonextraction cases. For a complete review of the pertinent literature, see Table 1. 29 Table 1. A Review of the Literature. Authors Uhde et al20 (n=72) Artun et al3 (n=78) Little et al8 (n=61) Glenn et al21 (n=28) Little et al22 (n=30) McReynolds et al. 23 (n=46) Haruki et al24 (n=83) Erdinc et al19 (n=98) Paquette et al18 (n=63) Boley et al25 (n=32) Shields et al16 (n=54) Peck & Peck26 (n=115) Sample Cl I & II Ext & Nonext Cl II Ext & Nonext Cl I & II Ext Cl I & II Nonext Cl I & II Ext & Serial Ext CL I & II Ext & Serial Ext Early vs. Late Tx Bicuspid Ext Cl I & II Ext & Nonext Cl II, Borderline Ext & Nonext Cl I Ext Cl I & II Ext Untreated Female Occlusions Shah et al27 (n=50) Untreated Occlusions (Male subgroups had NS value) Rhee et al28 (n=69) Puneky et al29 (n=77) Freitas et al30 (n=56) Edwards31 (n=194) Driscoll et al32 (n=87) Untreated Occlusions Cl I & II Ext & Nonext Cl I & II Ext Cl I & II Ext & Nonext Cl I & II Untreated Vs Ext Variable 1 Variable 2 Statistical Value Tx ∆ L3-3 PostTx ∆ L3-3 r = -0.38* Tx ∆ L3-3 PostRet II r = 0.314** Tx ∆ L3-3 PostRet AL PostRet II PostRet II PreTx AL PostRet AL r = 0.24 r = 0.52** r = 0.83, sample small no sig. value Serial Group (n=30); PostRet II Serial Group (n=14); PostRet II Early Tx Group (n=36); PostRet II Ext Group (n=49); PostRet II Ext PostTx ∆ II ABCH Matched Ext Group (n=30); PostRet II Ext Group (n=32); PostRet II Late Tx Group (n=47); PostRet II Nonext Group (n=49); PostRet II Nonext PostTx ∆ II L1 Movement Tx ∆L1/MP PostRet II r = 0.43* Tx ∆ L1 to NPg L2 MD/FL Perfect L2-2 MD & FL widths L1 MD1/MD2 Female (n=25) L2 MD1/MD2 Female (n=25) U & L Incisor width ratio at C & I PostRet ∆ L1 to NPg PostRet II Nonperfect L2-2 MD & FL widths r = -0.58** r = 0.31** Compared means, NS Compared means, NS Compared means** Compared means, NS Compared means, NS r = -0.75** Compared means** II r = 0.52** II r = 0.55** II r > 0.74** L1 & L2 MD/FL Long-term PostTx II r < 0.22 L1 & L2 MD/FL PostTx ∆ II r < 0.13 Control Group (n=93); PostRet II Untreated Group; ∆ II CSF Tx Group (n=101); PostRet II Ext Treated Group; ∆ II Compared means** Compared means, NS Key: Cl=Class; Ext=extraction; Nonext=nonextraction; Tx=Treatment; PostTx=posttreatment; PostRet=postretention; ∆=difference; * P<0.05; ** P<0.01; NS=not significant; L=lower; U=upper; II=incisor irregularity; AL=arch length; ABCH=apical base change/ difference in mandibular growth relative to the maxilla; MD/FL=mesiodistal width and faciolingual width ratio; MD1=mesiodistal width at the incisal; MD2 mesiodistal width at the gingival; C & I=cervical and incisal; CSF=circumferential supracrestal fiberotomy 30 Materials and Methods Sample This retrospective study consisted of diagnostic records from 62 cases, from the Saint Louis University archive, with Class I (n=38) or Class II (n=24) malocclusions and posttreatment irregularity index of no more than 0.5 mm. A high quality posttreatment index was required to allow detection of any minor incisor changes that may occur after treatment. Records are from pretreatment, posttreatment, and one year posttreatment. Models were required to have all their lower six anterior teeth. Cases with Class III malocclusion, mutilated dentition, craniofacial anomalies, fixed lower retainers, dental restorations on the first anterior teeth, or a history of orthognathic surgery were excluded from the study. The sample consisted of a mixture of females (n=40) and males (n=22), all within the age range from 9 to 18 years old at the time of treatment. Treatment modalities included all forms such as extraction, nonextraction, Tipedge mechanics, and Tweed mechanics. 31 Cephalometric Analysis Cephalometric records for each patient were evaluated at pretreatment (T1), posttreatment (T2), and one year posttreatment (T3). Each lateral cephalometric radiograph was traced by a single operator with a number 2 lead pencil on acetate paper. The lower incisal angle was measured from a line through the long axis of the most anterior incisor on the radiograph and a mandibular plane (line) drawn from menton to an anatomical gonion. The anatomical gonion is a point on the mandibular angle estimated by a bisecting line of the angle constructed from the posterior ramus and mandibular inferior border. The incisor to mandibular plane angle is designated as IMPA. Model Analysis Models from pretreatment (T1), posttreatment (T2), and 1 year posttreatment (T3) were measured with an electronic caliper calibrated to the nearest hundredth by a single operator. Irregularity index (II) measurements recorded, as described by Little7, a summation of the displaced contact points between the lower six anterior teeth (Figure 1). 32 Figure 1. Irregularity index (II) defined as the summed displacement of adjacent anatomic contact points of the mandibular anterior teeth. Statistical Analysis The analysis of error was performed by measuring 10 records 1 week apart by the same. The intraclass correlation coefficient for this single operator measuring the irregularity index was 0.9920 and for the lower incisor to mandibular plane angle was 0.969 (Table 2). Means and standard deviations were calculated for all variables. Independent t-tests were used to compare means between subgroups. Significance was determined at p<0.05. Pearson correlation was used to calculate correlation coefficient between two variables. 33 Table 2. Analysis of Error IMPA IMPA' Irregularity Irregularity' 89 86 0.4 0 89 91.5 0.8 0.8 100.5 99.5 0 0 100 100 0.8 0.8 85 84 0.2 0.3 93 90 1.3 1.2 83 82.5 0.4 0.78 94 97 0.4 0.4 99.7 99.7 1.4 1.62 100 99 3.53 4.02 ICCIMPA 0.979* ICCII 0.987* *p < 0.001; ICC= intraclass correlation Results Our sample consisted of 62 cases with a mean posttreatment incisor irregularity of 0.17±0.19 mm, age=12.64±1.72 years, treatment time=2.16±0.58 years. Other averages are displayed on Table 3. The pretreatment IMPA range was 72.4° to 112° and the posttreatment IMPA range was 81.5°-111°. Significant difference of the means was observed between the Class I and Class II malocclusion in pretreatment (T1) IMPA and the treatment change (T2-T1) in IMPA. Significant difference in mean posttreatment 34 irregularity index was observed between male and female patients, 0.13 mm and 0.25 mm, respectively. Table 3. Mean ± SD of the Sample and Subgroups. ♀ (n=40) Measurement Cl I (n=38) Cl II (n=24) T1 II 3.78±2.55 4.17±2.67 3.91±2.85 T2 II 0.16±0.18 0.18±0.20 0.13±0.17* T3 II 0.68±0.91 0.74±0.55 0.75±0.89 T1 IMPA 91.51±6.87* 95.35±6.43* 93.21±7.70 T2 IMPA 93.91±7.47 97.45±6.81 95.57±7.78 T2-T1 IMPA 5.63±4.02* 3.05±3.56* 4.88±4.50 *Significance, P<0.05 ♂ (n=22) 3.97±2.08 0.25±0.19* 0.62±0.56 92.61±5.32 94.75±6.71 4.18±3.01 Overall 3.93±2.59 0.17±0.19 0.70±0.79 93.00±6.91 95.28±7.37 4.63±4.02 Correlation analysis reveal significant correlation (r=0.328, p<0.01) between the lower incisor changes during treatment (T2-T1) and lower incisor irregularity one year after treatment (T3-T2). To test Dr. Tweed’s philosophy33 of a stable lower incisal angle between the ranges of 85° to 93°, t-test analysis was used compared incisor irregularity of cases (n=22) completed within Tweed’s range and those cases (n=40) that were out of the range. A statistic comparison between the two groups is shown on Table 4. No significant differences were found between the cases with T2 IMPAs within Tweed’s range and cases that did not. In addition, to maintain approximately equal sample size and compare incisor irregularity means with the change in lower incisor angle, the cases were divided into a group (n=30) with ≥ 4° change to the lower incisor angle during treatment 35 and a group (n=32) with < 4° change in lower incisor angle. A statistical analysis of the groups can be seen on Table 5. No significant difference was observed between the mean lower incisor irregularities of the groups before treatment and one year after treatment. Table 4. Comparison between Groups within Tweed Norms (85°-93°) and Other IMPAs T1 II Subgroups Tweed N 22 Mean 3.71 Other 40 4.05 Std. Deviation Std. Error Mean 2.52 0.54 2.65 0.42 Tweed 22 0.13 0.17 Other 40 0.19 0.20 T3 II Tweed 22 0.55 0.51 Other 40 0.79 0.90 T1 IMPA Tweed 22 89.73 5.03 Other 40 94.80 7.19 T2 IMPA* Tweed 22 89.89 2.66 Other 40 98.24 7.46 T2-T1 IMPA Tweed 22 3.66 2.72 Other 40 5.17 4.53 Significant difference (p<0.01) are noted with an (*) 0.04 0.03 0.11 0.14 1.07 1.14 0.57 1.18 0.58 0.72 T2 II Table 5. Comparison between Groups with Incisal Changes < and ≥ 4° Subgroups N Mean Std. Deviation Std. Error Mean T1 II 32 4.38 2.57 < 4° 30 3.45 2.55 ≥ 4° T2 II 32 0.17 0.19 < 4° 30 0.18 0.18 ≥ 4° T3 II 32 0.66 0.55 < 4° 30 0.75 0.99 ≥ 4° T1 IMPA 32 93.95 7.10 < 4° 30 91.98 6.67 ≥ 4° T2 IMPA 32 95.07 7.07 < 4° 30 95.50 7.79 ≥ 4° T2-T1 IMPA* 32 1.74 1.05 < 4° 30 7.72 3.71 ≥ 4° Significant difference (p<0.01) are noted with an (*) 36 0.45 0.47 0.03 0.03 0.10 0.18 1.26 1.22 1.25 1.42 0.19 0.68 Discussion Weinberg and Sadowsky17 concluded from their study that the resolution of crowding is achieved by generalized expansion of the buccal segments and advancing the lower incisors. Previous studies3,16,25,34 have suggested that proclined incisors tend to relapse after treatment. Proclining the lower incisors can also effectively increase arch length. Previous studies8,19,23 have also shown that arch length decreases over time following orthodontic treatment. Together these data support the concept that changing the lower incisal angle may have some consequential effect on crowding of the mandibular incisors. However, the current study suggests that changing the lower incisor angulation contributes to postreatment lower incisor crowding, but this relationship is very variable. As indicated by the literature, stability is multifactoral2,3,11,15,16,30,35; therefore variability is expected. Schulhof et al.36 (1977) did a study comparing the change in lower incisal edge in relation to a perpendicular to the Frankfort horizontal and the absolute arch discrepancy. From a total sample of 78, they found no statistical difference between the change in lower incisor 37 position and stability. The study compared 3 groups: group I (n=23) with lower incisors moved forward 1 mm or more, group II (n=26) with lower incisors finished between 2 mm lingual and 1 mm labial of their original position, and group III (n=29) with the lower incisors retracted 2 mm or more. Schulhof’s data is consistent with our findings. We found no difference in instability whether the lower incisor angle is changed ≥4° or <4° from its original position. In a recent article by Janson and associates37, it was showed by statistical analysis that a compromised upper bicuspids extraction treatment for a severe Class II malocclusion with a large overjet is not any less stable than 4 bicuspids extraction. The authors concluded that proclining the lower incisors does not make the treatment less stable. Unfortunately, the authors show no statistics to recommend how much proclination is still acceptable; the present study shows no statistical significant difference between the incisor irregularities of cases with a change in the lower incisor angle ≥4° and <4°. This implies that the stability is the same whether the lower incisal angle is proclined more than 4° or not. However, as illustrated on the scattergram (Figure 2) of 38 the lower incisal angle change and irregularity index, we observe greater variability in irregularity when lower incisor angle is changed more than 4° during treatment. 4.5 Irregularity Index (mm) 4 3.5 3 2.5 2 1.5 1 0.5 0 0 2 4 6 8 10 12 14 16 18 IMPA Change (degrees) Figure 2. Correlation Between the Absolute Change in Lower Incisal Angle and Irregularity Index at 1 Year Posttreatment. n=62; r=0.328; p<0.05 The data of this study is also consistent with a study38 that evaluated proclined incisors on adults with surgically treated mandibular prognathism; the author found no difference in irregularity index between a group (n=29) with more than 10° proclined lower incisors and a group (n=33) with minimal change in incisal inclination. In discussion, the author38 states that prerequisites to significantly proclining the incisors are: (1) the skeletal malrelationship is corrected surgically and (2) 39 inclinations of the teeth are within the range of established norms. We tested the Tweed norms and found no statistical difference in crowding between the group with posttreatment IMPA within Tweed norms (85° to 93°) and the group outside of Tweed norms. Our incisors were significantly tipped outside of the Tweed norms without any surgical correction. This suggests that the lower incisor normal range might be larger than what Tweed33 suggested or placing the lower incisor out of Tweed’s range does not significantly affect posttreatment stability. Lenz and associates39 examined the change in incisal position during treatment of 55 patients and its relation to postretention PAR scores. They found no significant relation with the PAR score to any of the incisal measurements (interincisal angle, L1-APo, L1-GoGn, and U1PP). In contrast to the current study, we found a significant (p<0.01) correlation coefficient (r=0.328) between the IMPA and one year posttreatment irregularity index. The difference in findings suggests that the lower incisal angle may not have any or little influence in the stability of the posterior occlusion, which is accounted for in the PAR score. Limitations 40 One limitation of this study is that the irregularity index is not representative of a malocclusion where the incisors may have normal contacts interproximally but are aligned in a zigzag pattern. Anterior teeth with perfect contacts but aligned in zigzag pattern would technically record as 0.0 mm for its irregularity index. Another limitation is that the posttreatment time evaluated was relatively short and the posttreatment irregularity does not truly reflect the actual instability of the malocclusion. There was no way to control for the amount of time that each patient wore their retainers. However because the groups where randomly selected with regards to retainer wear time, this factor will statistically cancel out any differences that it may have on our results. Conclusion To summarize, it can be concluded that: • Changing the lower incisial angle is significantly related to postreatment lower incisal crowding, r=0.328 (P<0.01). • There is no statistically significant difference in stability whether the lower incisor changes were beyond 4 degrees or not, but more variability in 41 posttreatment crowding was observed when the lower incisor was changed beyond 4 degrees. • Cases finished within Tweed’s lower incisal range (85°93°) did not have more stable results than cases that were outside this range. 42 Literature Cited 1. Graber TM, Vanarsdall RL. Orthodontics: Current Principles and Techniques. St. Louis: C.V. Mosby; 2000. 2. Shah AA. Postretention changes in mandibular crowding: a review of the literature. Am J Orthod Dentofacial Orthop 2003;124:298-308. 3. Artun J, Garol JD, Little RM. Long-term stability of mandibular incisors following successful treatment of Class II, Division 1, malocclusions. Angle Orthod 1996;66:229238. 4. 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Trans Eur Orthod Soc 1972:113-122. 11. Richardson ME, Gormley JS. Lower arch crowding in the third decade. Eur J Orthod 1998;20:597-607. 12. Bishara SE, Treder JE, Jakobsen JR. Facial and dental changes in adulthood. Am J Orthod Dentofacial Orthop 1994;106:175-186. 13. Little RM. Stability and relapse of mandibular anterior alignment: University of Washington studies. Semin Orthod 1999;5:191-204. 14. Littlewood SJ, Millett DT, Doubleday B, Bearn DR, Worthington HV. Orthodontic retention: A systematic review. J Orthod 2006;33:205-212. 15. Ormiston JP, Huang GJ, Little RM, Decker JD, Seuk GD. Retrospective analysis of long-term stable and unstable orthodontic treatment outcomes. Am J Orthod Dentofacial Orthop 2005;128:568-574; quiz 669. 16. Shields TE, Little RM, Chapko MK. Stability and relapse of mandibular anterior alignment: a cephalometric appraisal of first-premolar-extraction cases treated by traditional edgewise orthodontics. Am J Orthod 1985;87:27-38. 17. Weinberg M, Sadowsky C. Resolution of mandibular arch crowding in growing patients with Class I malocclusions treated nonextraction. Am J Orthod Dentofacial Orthop 1996;110:359-364. 18. Paquette DE, Beattie JR, Johnston LE, Jr. A long-term comparison of nonextraction and premolar extraction edgewise therapy in "borderline" Class II patients. Am J Orthod Dentofacial Orthop 1992;102:1-14. 19. Erdinc AE, Nanda RS, Isiksal E. Relapse of anterior crowding in patients treated with extraction and 44 nonextraction of premolars. Am J Orthod Dentofacial Orthop 2006;129:775-784. 20. Uhde MD, Sadowsky C, BeGole EA. Long-term stability of dental relationships after orthodontic treatment. Angle Orthod 1983;53:240-252. 21. Glenn G, Sinclair PM, Alexander RG. Nonextraction orthodontic therapy: posttreatment dental and skeletal stability. Am J Orthod Dentofacial Orthop 1987;92:321-328. 22. Little RM, Riedel RA, Engst ED. 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Tooth morphology and lower incisor alignment many years after orthodontic therapy. Am J Orthod 1984;86:299-305. 45 30. Freitas KM, de Freitas MR, Henriques JF, Pinzan A, Janson G. Postretention relapse of mandibular anterior crowding in patients treated without mandibular premolar extraction. Am J Orthod Dentofacial Orthop 2004;125:480487. 31. Edwards JG. A long-term prospective evaluation of the circumferential supracrestal fiberotomy in alleviating orthodontic relapse. Am J Orthod Dentofacial Orthop 1988;93:380-387. 32. Driscoll-Gilliland J, Buschang PH, Behrents RG. An evaluation of growth and stability in untreated and treated subjects. Am J Orthod Dentofacial Orthop 2001;120:588-597. 33. Tweed CH. Indications for the extraction of teeth in orthodontic procedure. American Journal of Orthodontics and Oral Surgery 1944;30:405-428. 34. Cole HJ. Certain results of extraction in the treatment of malocclusion. Angle Orthod 1948;18:103-113. 35. Zachrisson BU. Mandibular third molars and late lower arch crowding--the evidence base. World J Orthod 2005;6:180-186. 36. Schulhof RJ, Allen RW, Walters RD, Dreskin M. The mandibular dental arch: Part I, lower incisor position. Angle Orthod 1977;47:280-287. 37. Janson G, Busato MC, Henriques JF, de Freitas MR, de Freitas LM. Alignment stability in Class II malocclusion treated with 2- and 4-premolar extraction protocols. Am J Orthod Dentofacial Orthop 2006;130:189-195. 38. Artun J, Krogstad O, Little RM. Stability of mandibular incisors following excessive proclination: a study in adults with surgically treated mandibular prognathism. Angle Orthod 1990;60:99-106. 46 39. Lenz GJ, Woods MG. Incisal changes and orthodontic stability. Angle Orthod 1999;69:424-432. 47 Vita Auctoris Binh Tran was born on February 2, 1975 in Vietnam. migrated to the United States in 1980. He He attended the University of California, Los Angeles (UCLA) from 19931998. While at UCLA, he participated in a study abroad program in Beijing, China for the summer and winter quarters of 1996. After graduating from UCLA with a B.S. in Physiology, he spent a year teaching at Roosevelt Middle School in Oakland, California at the same time continued to do research studies which he began as an undergraduate at UCLA in 1997. He met his wife, Michele Ma, in April of 1997 and they married in December 16, 2000. He entered dental school at the University of Southern California in 2000 and receive his D.D.S. degree in 2004. During dental school, the following papers were published: 1. Yee, H.F., Melton, A.C., and Tran, B.N. Rho A/Rho-Associated Kinase Mediates Fibroblast Contractile Force Generation. Biochemical and Biophysical Research Communications, Feb. 2001: Vol 280, 1340-1345. 2. Saab, S, Tam SP, Tran BN, Melton, AC, Tangkijvanich, P, Wong HC, Yee, Jr HF, Myosin Mediates Contractile Force Generation by Hepatic Stellate Cells in Response to Endothelin-1, Journal of Biomedical Science, 2002;9:607-612. 3. *Kernochan, L, *Tran, B, Tangkijvanich, P, Tam, SP, and Yee, Jr HF, Endothelin-1 Stimulates Human Colonic Myofibroblast Contraction and Migration, GUT, January 2002, Vol 50, No 1, 65-70. *These authors contributed equally to this study Following graduation from dental school, he began his studies at Saint Louis University, in pursuit of a Master’s Degree from the Orthodontics program. Following graduation on January 2007, he plans to move with his wife and two 48 children, Maximus and Audrey, to northern California to begin work as an orthodontist. 49